The only noteworthy one out of those is the call from module_invoke(), which in turn is called a lot. But will this additional call to module_exists() be measurable? I guess not, but obviously we need to bench it.

For unrelated reasons, I recently tested in d6 calling function_exists vs module_exists to see which was faster and discovering function_exists was better by an extreme amount. Adding a module_exists to what current a function_exists does make it quite heavy, I think. Does anywhere call module_hook on disabled modules?

node_hook() supports a strange fake-hook-naming pattern based on the "base" property of a node type, which is 'node_content' for all custom node types created via Node module's UI. But 'node_content' is not a module that exists.

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